The present invention concerns the servicing of aircraft landing gear shock absorbers. More particularly, but not exclusively, this invention concerns a method of servicing a shock absorber on a landing gear assembly of an aircraft, and an apparatus for servicing such a shock absorber. The invention also concerns an associated control unit for use in such a method or apparatus and an associated computer program product.
Aircraft landing gear assemblies typically include a shock absorber assembly (for example of the type referred to as an oleo strut) which provides suspension and shock absorbing functions for the aircraft during landing and take-off. An aircraft 101 including such shock absorber assemblies 102 is shown in FIG. 1. A single such shock absorber assembly 102 is shown in greater detail in FIG. 2 together with a schematic illustration of the wheels 109. (Parts of the landing gear including, for example, the axles for mounting the wheels and the upper part of the landing gear for receiving the oleo strut, have been omitted from FIG. 2 for the sake of clarity.) The shock absorber 102 comprises a piston 104 received within a cylinder 103, as is conventional with such shock absorbers. The cylinder 103 and piston 104 are connected via a torque link 107 for inhibiting rotation of the piston 104 relative to the cylinder 103, as is well known to those skilled in the art. The shock absorber 102 typically includes one or more gas springs (not shown separately in FIG. 2) with damping provided by hydraulic fluid (also not shown separately in FIG. 2). Over time, as the shock absorber 102 is used on successive landings and take-offs, a proportion of the gas and/or the hydraulic fluid escapes. It is important to ensure that the levels of both the gas and the hydraulic fluid are correct, as otherwise performance of the shock absorber may be adversely affected. Regular servicing of the shock absorber is thus required to ensure that the gas and fluid levels in the shock absorber are maintained at suitable levels.
Accurate determination of the levels of the gas and the hydraulic fluid is not however a straightforward process. The amount of gas in the shock absorber is typically measured by means of measuring the extension of the shock absorber relative to a reference point (often referred to as ‘H-dimension’) with the weight of the aircraft being supported by the landing gear assemblies of the aircraft. FIG. 3 shows an H-dimension (labelled as “h”) as being measured between the bottom of the sliding cylinder 103 and the top of the attachment lug of the lower torque link 107 of the shock absorber 102. The temperature and pressure of the gas are then used to assess, using a look-up chart, whether the “H-dimension” indicates that there is an appropriate amount of gas in the shock absorber. The gas temperature thus needs to be measured. Often a sensor measures the gas pressure, which is of course proportional to the load supported by the shock absorber. Thus, according to a typical servicing procedure, the pressure and temperature are ascertained and the H-dimension of the shock absorber is measured; if, for that given pressure and temperature, the H-dimension is below a defined limit, then one of the following maintenance actions takes place:                1. Weight on wheels service: The shock absorber is inflated with gas (typically Nitrogen, N2) to raise the H-dimension of the gear to above the limiting threshold.        2. Weight off wheels service: The aircraft is jacked and the shock absorber is depressurised. The shock absorber is redressed with hydraulic fluid and subsequently charged with N2 to a specified pressure.        
The use of the H-dimension to assess incorrect levels of gas and hydraulic can suffer from inaccuracies resulting from the friction between the moving parts of the shock absorber.
There are also disadvantages with both of the abovementioned procedures for servicing of landing gear shock absorbers. With a weight on wheels service, it is often assumed that the reduction in the H-dimension is the result of leakage of N2 alone from the shock absorber. There are other reasons why the H-dimension might be lower than desired, including for example the loss of hydraulic fluid. Whilst some shock absorber designs permit weight on wheels servicing of both N2 and hydraulic fluid, such a servicing process is in practice a complicated and demanding process for a maintenance engineer, and one which can increase the risk of the shock absorber being serviced with relatively low accuracy. The process may take one or two hours per shock absorber. A weight off wheels service on the other hand is more likely to result in an accurate and correct refilling of the shock absorber with the hydraulic fluid and gas. However, a weight off wheels service involves jacking-up of the aircraft and can take one or two days to complete. This is costly to the operator as the aircraft will be taken out of revenue service whilst the maintenance action is carried out. Thus, whilst a weight-on-wheels service is fast, the confidence of refilling/topping up the levels of hydraulic fluid and gas accurately to the correct level is not as great as the much slower and more costly weight-off wheels service. In both cases, the maintenance engineer may have to follow a large number of manual actions to complete the service. There is therefore an associated risk of human error resulting in an incorrectly serviced gear.
GB2514336 describes a method for carrying out a weight-on-wheels service of a landing gear shock absorber, in which the levels of damping fluid and gas are ascertained and then adjusted as required using various sensors and calculations. It is believed that further improvement is however possible.